A key requirement in the development of analytical sensor systems for industrial, medical, security, and domestic applications is their ability to promptly and reliably detect and recognize a broad range of analytes, often in low concentrations, while working in a continuous mode.
It is well-known that materials or chemical processes release characteristic complex gas ensembles that can be used like a fingerprint for condition monitoring. These complex gas ensembles are generally referred to as odors if perceptible by the human nose. Sensor systems are being designed to mimic the data acquisition principles of mammalian olfactory systems, which allow one to discriminate single gases as well as odor like gas ensembles by creating and processing a multidimensional pattern of many signals generated by a receptor (i.e. sensor) array. Devices employing this pattern recognition concept are generally referred to as electronic noses.
The KAMINA (Karlsruhe Research Center) platform is one example of a multielectrode nose. See U.S. Pat. No. 5,783,154. The key feature of the KAMINA technology is the substitution of an ensemble of separate conventional sensors with the gradient technique applied across the sensing elements. Conventional KAMINA technology applies the gradient to a single metal oxide layer deposited onto an array of Pt electrodes, with adjacent Pt electrodes forming a sensing element. Two gradients are typically applied across the sensor to differentiate the response of the sensing elements: (i) a lateral variation of surface temperature of the film; and (ii) a gradual thickness change of a gas permeable coating topping the metal oxide layer.
Recent developments in micro- and nanotechnologies have made available new material platforms, device fabrication alternatives, and novel sensing concepts to improve sensitivity, reliability, energy consumption, and response time of sensors. For example, quasi-one-dimensional metal oxide nanostructures have been found to be well suited for sensor applications because a multitude of sensing properties are substantially improved compared to compact metal oxide gas detecting elements. Namely, the high surface-to-bulk ratio of the quasi-one-dimensional metal oxide nanostructures allows very sensitive transduction of the gas/surface interactions (adsorption and catalytic oxidation) into a change of electrical conductivity. The radius of these nanostructures approaches the material's Debye length, which makes nearly the entire nanostructure a depletion or accumulation zone of mobile charge carriers in response to surface redox process, and thus establishes an extreme sensitivity of the electron or hole transport to charge transfer interactions of gas molecules at the surface. In addition, the nanostructures ability to accept a variety of morphologies and structures in conjunction with their surface and bulk doping offers wide possibilities to tune the gas-sensing properties. The contacts between the grains of conventional granular film sensing elements have propensity to sinter with time and therefore reduce stability of the sensor. Single crystal metal oxide nanowire mats have elevated resistance toward this phenomenon. Advantageously, the quasi-one-dimensional nanostructures render the empty space between adjacent nanostructures no matter how small the diameter of the individual nanowire, as compared to nanostructured oxide films, which hamper the gas diffusion with reduction of the grain size. There exists however a technological gap between the laboratory demonstrations using quasi-one-dimensional nanostructures as analyte sensors and a practical electron nose microdevice suitable for up to date, large scale microfabrication and capable of operating in real-world conditions environments.